Aerosols from natural, anthropogenic and industrial sources have long been recognized as a potential threat to human health; to that list of sources we now must add airborne chemical or biological warfare agents as a source of potentially lethal exposure or terrorist threat. Effective sampling and collection of aerosol particles is a critical first step in the detection and identification of these hazards. Identification methods such as immunological or nucleic acid assays typically require the aerosol sample to be suspended in a liquid medium. There is therefore a need for a “front-end” device adapted to collect these aerosols and prepare or deliver them in a concentrated suspension or solution for analysis.
Higher concentration of aerosol particles in a liquid sample achieves greater sensitivity for many analyses. Today's micro analytical instruments require microliter or nanoliter sample volumes, and larger volumes of liquid are difficult to process. Moreover, currently available aerosol collectors cannot readily be adapted to perform sample preprocessing prior to analysis, a significant disadvantage for integrated sampling and detection platforms. Sample preprocessing includes processes such as dissolution of sample matrix, lysis of suspect cellular contents, or preliminary screening to trigger more exhaustive analysis, process steps which speed threat detection and avoid unnecessary analyses.
Current aerosol collection devices that provide liquid samples for analysis, such as wetted wall cyclones, wetted rotating vane impactors, and liquid impingers, are inefficient and produce large volume liquid samples, and are not well adapted to concentrating an aerosol. Large sample volumes are suitable for use with assays using 96- or 384-well plates, but the current trend is towards smaller scale, more-automated procedures using microfluidics, which demand much smaller sample volumes. Liquid impingers and wetted wall collectors of the prior art cannot simply be miniaturized because of the drying effects of evaporation during operation and the difficulty in holding in place a small volume of a liquid under a stream of high velocity gas.
For example, in US Pat. Appl. Doc. No. 2004/0232052 to Call, a “liquid jet” (see FIGS. 19 and 20A of US 2004/0232052) is applied so that samples are “blasted off the collection surface” (p. 22, para. 0238) into a sample container. Such a procedure can only result in losses of sample and increased dilution, and is likely not workable.
Thus there remains a need for a collector capable of efficiently concentrating an aerosol from a large volume of air into a few microliters or nanoliters of a liquid sample. In this regard, the field of microfluidics has revolutionized many aspects of chemistry and microbiology and is an enabling technology for the development of a wide range of detection and identification methods. Following the pioneering work of Wilding (U.S. Pat. Nos. 5,304,487, 5,376,252, 5,726,026, 5,955,029, 6,953,676), continuous and intermittent-flow microfluidic devices have been developed that carry out nucleic acid and immunological analyses in integrated devices fabricated on silicon or glass substrates. Digital microfluidic devices employ technologies such as electro-wetting, diaelectrophoresis, or microhydraulics to move, mix, combine and split microliter and nanoliter volumes and allow chemical and biological assays to be automated and carried out at very small scales. These advances offer substantial advantages in speed and accuracy while greatly reducing the need for operator involvement and minimizing reagent volumes. However, the problem of developing an effective “front-end” interface for ambient aerosol particle concentration, collection, and delivery in a nano- or microvolume to “back-end” analytical instrumentation has not been addressed.
Aerosol pre-concentration, prior to sample collection, offers a significant advantage when coupled to an analytical method. Using a variety of devices known in the art as “virtual impactors”, aerosol particles to be sampled from a larger volume of air are concentrated into a particle-enriched gas stream of smaller volume (the “minor flow” or “particle-enriched flow”) while the bulk of the sampled air, depleted of particles, (also termed the “major flow”, “bulk flow”, or “particle-depleted flow”) is discarded. Such an aerosol concentrating device is described in US Pat. Appl. Doc. No. 2008/0022853, entitled “Aerodynamic Lens Particle Separator”, and is co-assigned to the Applicant. Other air-to-air concentrators include virtual impactors such as the US Army's XM2 virtual impactor, those described in U.S. Pat. Nos. 3,901,798, 4,670,135, 4,767,524, 5,425,802, 5,533,406 and 6,698,592, and others.
An aerosol-to-liquid collection and delivery system that accepts raw or concentrated aerosols and delivers resuspended or solubilized aerosol particles in small droplets of fluid will serve as the front end to a number of biochemical or physical detection platforms. Initial demand is expected to be primarily in the security, military, and biomedical fields, but also in environmental and industrial sampling and monitoring applications, and will be driven to smaller sample volumes by technological advances in the development and integration of detection platforms and assays, including and not limited to both in situ and downstream assays for particles and particle constituents.
However, particle traps become fouled with accreted deposits when overloaded in extended use and typically are protected by upstream filters that prevent entry of oversized materials such as dust, fibers, or aerosolized salt crystals which would otherwise block gas flow. Accumulation of micron- or submicron-sized particles can also result in blockage. As a result, these devices must be continuously monitored for performance, for example by monitoring backpressure and/or continuity of flow. This problem has adversely impacted the wider use of particle traps for a variety of industrial and security applications in favor of particle collection devices that rely on wetted wall or liquid impingement technology, both of which are comparatively less sensitive and less portable.
Similarly, “air-to-air aerosol concentrators” such as aerodynamic lenses and virtual impactors, which are frequently used to fractionate and concentrate particles in a gas flow prior to collection or detection, are also hampered by fouling considerations. For example, particle deposits can accumulate around the mouth of a virtual impactor, often termed a “skimmer”, where the gas flow is split into a “minor flow” enriched in particles and a “major flow” (sometimes termed “bulk flow”) depleted of particles. Particle accumulation on the surfaces of these devices, particularly on surfaces and in channels around the skimmer mouth, unacceptably alters device performance. These are not the intended particle collection surfaces, but nevertheless become progressively fouled. Unfortunately, deterioration of performance accelerates over time: i.e., as deposits become larger the fouling rate increases in a vicious cycle.
Removal of particle deposits can be technically difficult. The channels used in inertial impactors may be looped and have small dimensions. A mechanical arm such as a pipe cleaner inserted into the channel to clean the channel must be thin and flexible, and excess force in cleaning can result in formation of a packed mass that cannot be physically removed. Disassembly for cleaning, such as by removal of cathodic and anodic plates of an electrostatic precipitator, can be inconvenient or not possible. Aggressive chemical cleaning solutions can damage the smoothness of the channel surfaces. Also, aggressive cleaning methods will likely result in destruction of the structure and/or composition of the captive particles, defeating a basic purpose of particle collection and sampling for analysis, and hence are not satisfactory.
Call, in U.S. Pat. No. 6,938,777, describes a method for removing concentrated “spots” of deposited particles from an impactor surface, which involves first transporting the surface bearing the spot from the collection device, and then subjecting the spot to a blasting jet of fluid or using a mechanical scraper to dislodge the spot. In practice, the impactor surface is formed on a moveable solid support, for example in the form of a roll of tape or a rotating disk, so that the impactor surface can be translocated from the collection apparatus, and the method is thus not generally applicable. Where the internal surface cannot be removed from the aerosol collector, such as the internal surfaces of particle traps or virtual impactors or for sampling of particles entrapped within an enclosed particle trap, no solution is provided.
In U.S. Pat. No. 7,578,973, Call goes on to point out that particulate “wall loss,” i.e. unintended deposition of particles on various surfaces of virtual impactor structures (especially the curved or bent portions) remains a challenging problem (Col 2, lines 24-36).
Thus, there is a need in the art for a method and apparatus to clean particle traps and aerosol concentrators such as virtual impactors, either in response to a change in gas flow associated with accumulation of particles therein, or periodically as prophylaxis against accumulation of particles. Preferably the method also facilitates particle sampling.